1. Technical Field
This invention relates to cordless telephones and more particularly to cordless telephones arranged for deployment in a public telephone system.
2. Description of the Prior Art
Telephone communications in public areas have traditionally been limited to a person using a public telephone where he or she could make either charge calls, pay calls or credit card calls. The mobility of the user has thus been limited to the immediate surroundings of the public telephone, as determined by the length of the telephone handset cord. Other alternatives, such as cellular telephone service and the new telepoint system, are expensive and therefore unattractive to many potential users.
One economical alternative to the public telephone is a public cordless telephone system. Such a system is described in U.S. Pat. No. 4,953,198 which issued to J. J. Daly et al. on Aug. 28, 1990. The public cordless telephone system includes a plurality of public base units that are accessible by a plurality of portable, or cordless, telephone handset units. Each one of these handset units is capable of establishing communications with each one of the base units over a plurality of predetermined channels. Communication over each one of these channels as by way of a radio frequency carrier signal that is modulated by control signals and audio signals to and from a public base unit as appropriate. And the public base units are connected to telephone lines for providing convention telephone communications for the handset units.
In the operation of the public cordless telephone system, a communications link between the handset unit and the public base unit is established by a user activating the handset unit which, in turn, interrogates each one of a plurality of predetermined channels in order to establish communications with a public base unit on a nonbusy, or available, one of those channels. Once the handset unit selects an available channel, it transmits a service request which includes an identification code over this channel. A public base unit within the reception range of the handset unit receives the handset unit's identification code, appends its own identification code to this received code and then transmits the combined handset unit identification code and public base unit identification code back to the handset unit. The handset unit receives this combined code from the public base unit and then compares the handset portion of the received code with the code that it previously transmitted. If a favorable comparison of this code and the handset portion of the code received from the public base unit is obtained, the communications link is then established between the handset unit and the public base unit on that selected channel.
As public base units increase in number, many service providers are expected to provide carrier service for public base units. Thus if a handset unit happens to be within the reception range of two or more public base units that are capable or responding to the handset unit on the same channel, either one of these base units may respond to and establish communications with the handset unit. Since these public base units may not be provided by the same service provider, certain desirable communication options or features available from one service provider may not be available from the other.